What this is
A pale sour wort designed to be brewed over and over until a barrel is full. This is the unit batch, not the beer — the beer is what comes out of the foeder a year later.
The recipe is deliberately plain: two malts and oats, one small hop charge, no character to speak of. Everything interesting happens in the oak.
Brewing to fill a barrel
A 115 L foeder takes around 130 L to brim once you account for what the wood absorbs, which is roughly six of these. They are brewed as 11 kg double-batch grists across a single 50 L boil, split into two ~23 L cubes, so each brew day yields two units.
Consistency matters more than usual here. Every cube goes into the same vessel and blends with everything already in it, so a batch that misses its numbers does not make one bad beer — it moves the whole barrel. Brew the same wort every time and let the barrel do the varying.
The 2026 Lark barrel fill took seven units in total. One 23 L batch was pulled aside and fermented separately on the Sixth Magic culture rather than going into the foeder, which is the one intentional departure in the series.
Why the wort is this simple
18 IBU and no late hops. Bitterness is an antimicrobial control in a sour, not a flavour. Too much of it inhibits the Lactobacillus you are relying on, so the charge is a single 60-minute addition sized for stability rather than taste.
Mash at 65°C for 75 minutes. Long and low, for a fermentable wort. A mixed culture with Brett will eventually eat the dextrins a higher mash would leave, so leaving them in only means a slower, less predictable ferment rather than more body.
A quarter wheat and 9% oats for protein and mouthfeel that survive a year of attenuation down toward 1.002.
Pre-acidification
The lactic acid is the step that matters and the one most easily got wrong.
It goes into cold wort at barrel transfer, never into the hot cube — acidifying hot wort risks harshness and gives the culture no benefit. Target pH 4.0 to 4.2 before pitching, which is low enough to lock out most spoilage organisms while leaving the Lactobacillus room to work.
Add in 2 mL increments and measure between each with a calibrated probe. 8 to 10 mL is the expected range, not a dose.
Timeline
Expect a 12 to 18 month maturation once the barrel is brimmed. First sample around four months in, and be patient with a pellicle — a cold, slow barrel may not form one for a long time, and its absence is not a problem.